This invention relates to a novel method for coating a selected portion of the internal neck surface of a CRT (cathode-ray tube); and particularly the internal neck surface of a CRT having a beaded mount assembly.
A color television picture tube is a CRT which comprises an evacuated glass envelope including a viewing window which carries a luminescent viewing screen, and a glass neck which houses an electron-gun mount assembly for producing one or more electron beams for selectively scanning the viewing screen. Each gun comprises a cathode and a plurality of electrodes supported as a unit in spaced tandem relation from at least two elongated, axially-oriented support rods, which are commonly referred to as "glass beads." The beads have extended surfaces closely spaced from and facing the inner surface of the glass neck. The beads usually extend from the region close to the stem, where the ambient electric fields are small, to the region of the electrode to which the highest operating potential is applied and where the ambient electric fields are high during the operation of the tube. The spaces between the beads and the neck surfaces are referred to herein as "bead channels" and are channels in which leakage currents may travel from the stem region up to the region of the highest-potential electrode. These leakage currents in the bead channels are associated with blue glow in the neck glass, with charging of the neck surface, and with arcing or flashover in the neck. The driving field for these currents is the longitudinal component of the electric field in the channel.
Several expedients have been suggested for blocking or reducing the leakage currents. Coatings on the neck glass are reported to be effective in reducing the frequency of arcing at a given voltage or increasing the voltage at which flashover occurs. For example, U.S. Pat. Nos. 3,355,617 to J. W. Schwartz et al, 3,758,802 to T. Kubo et al and 3,979,632 to E. A. Gunning disclose, for flashover reduction, the use of an internal neck coating opposite the mount assembly and composed of metal oxides or crystallized glass. Also, Dutch patent application 67-05343, published Oct. 16, 1978, discloses, for flashover reduction, the use of an internal neck coating composed of vapor-deposited chromium metal that is at least partially oxidized by heating in air. Our own experiments have shown that the minimum voltage at which flashover occurs can be materially increased by using an internal neck coating of chromium metal that has been sputter-deposited in an argon-oxygen gas mixture. Each of the above-mentioned coatings is produced before the mount assembly is installed in the neck. While they may produce the desired effect in a CRT, a simpler, more reliable and less time-consuming method is desirable for factory production of color television picture tubes.